I'm making one too. It's actually picking up/rebooting a project I was working on in 2009, I showed some of my friends and they convinced me to not give up on it.
im going to warn you straight-up... If you want to make a GOOD farming/life sim, it's an incredible amount of work to undertake. There's a reason why I quit when I did in 2009, even though I was generally competent w/RM back then, it was still too complex for me. Simulation games have so many values processed under the hood. Even if you only use free resources made by other people, you have to think very creatively and pick scripts/plugins very judiciously in order to get it to function. Often times using them in ways they weren't expected to be used. Also be ready to encounter lots of bugs to fix.
for an example of what I'm talking about, this is basically what it looks like in the event editor to plant seeds:
Ex
If you want it to feel/act anything remotely like a Bokumono/HM game most of the eventing is going to look like that. If you just want to make a social sim like Animal Crossing (but presumably with humans and marriage) it may be more straightforward. Become as familiar as you can with variables and conditional branches.
as far as features go, you aren't ever going to please everyone so don't worry about that too much-- I say make a game with features you always wanted to see. Chances are other people will too.
also if you get frustrated with some feature you're having trouble adding, be patient! Sometimes you need to simply set it aside and work on something else, and suddenly an idea for a workaround will spring into your mind, and you try it-- and it works! (Not always, but don't feel too down if you have to try again, and again, and again... I think that's just part of the process of making a sim/non-RPGin RM...)